Democrats in Congress unveil a bill to rein in bias and excessive force in policing.

Democratic lawmakers in Congress introduced legislation on Monday aimed at ending excessive use of force by police officers across the country, and making it easier to identify, track and prosecute police misconduct.

The bill was introduced as a direct response to the recent killings of unarmed black Americans by police officers, as protests against police violence and racial discrimination continue across the country. It is the most expansive intervention into policing that lawmakers have proposed in recent memory.

The measure would curtail existing legal protections that shield police officers who are accused of misconduct from being prosecuted, and would impose new restrictions to prevent law enforcement officers from using deadly force, except as a last resort. It includes many proposals that civil rights activists have been pushing for decades against opposition from police unions and law enforcement groups.

“Never again should the world be subjected to witnessing what we saw on the streets in Minneapolis, the slow murder of an individual by a uniformed police officer,” said Representative Karen Bass, the chairwoman of the Congressional Black Caucus.

She was joined by Representative Jerrold Nadler of New York, the House Judiciary Committee chairman, and Senators Cory Booker of New Jersey and Kamala Harris of California in introducing the measure.

Immediately beforehand, the four lawmakers joined with Democratic leaders to honor George Floyd, a black man who was killed while being arrested by Minneapolis police. They kneeled for 8 minutes 46 seconds, the length of time an officer pressed his knee on Mr. Floyd’s neck.

Whether Democrats can seize the moment and push the changes into law remains unclear. They expect to pass the legislation swiftly in the Democratic-led House, but President Trump and Republican lawmakers who control the Senate have yet to signal which measures, if any, they would accept.

Mr. Trump has said little about the issue in recent days except to show his support for law and order on Twitter. In an interview with Fox News on Monday, Attorney General William P. Barr reiterated his view that police departments do not have a systemic problem with race.

Mr. Barr said that to draw conclusions about all police officers based on the actions of a few is akin to drawing conclusions about all black people based on negative stereotypes, which he called “one of the legitimate grievances of the African-American community.”

In the interview, Mr. Barr also contradicted Mr. Trump’s claim that he had visited a White House bunker for an inspection during the protests following Mr. Floyd’s death. “Things were so bad that the Secret Service recommended the president go down to the bunker,” Mr. Barr said. “We can’t have that in our country.”

Democrats in Congress will discuss their bill and hear testimony on police brutality and racial profiling at a House Judiciary Committee hearing on Wednesday. Among those set to testify is Philonise Floyd, a brother of George Floyd, according to a committee official familiar with the plans. The committee had yet to announce other witnesses.